The EPR spectrum of iron—sulphur centre B in photosystem 1 of Phormidium laminosum

Abstract

The photosystem I complex of plant photosynthesis contains bound iron-sulphur centres which appear to act as terminal electron acceptors for electrons released from P700 in the primary photoreactions [ 1,2]. The centres can be observed in the reduced state by low temperature electron paramagnetic resonance (EPR) spectroscopy. In photosystem I preparations from chloroplasts, and green and blue-green algae, the centre which is first reduced, either photochemically by illumination at low temperature, or by chemical reduction, has a spectrum at g = 2.05, 1.94, 1.86. This was termed centre A [3]. On further chemical or photochemical reduction the spectrum changes to a more complex one with features at g = 2.05, 1.94, 1.92 and 1.89 [4]. This change was interpreted as due to reduction of a second centre, B [3]. The g = 1.86 feature was assumed to have shifted due to an interaction with centre A. Intensity measurements indicated that centres A and B and P700 were present in equivalent amounts [2]. However the spectrum of reduced centre B alone could not be extracted by subtraction of the spectrum of centre A from the fully reduced spectrum. During EPR spectroscopic measurements of membrane preparations and photosystem I particles from the thermophilic blue-green alga Phormidium laminosum, we noted a signal at g = 2.065,1.935 and 1.882 (kO.002) in samples reduced in the dark with dithionite. Chemical and photochemical reduction experiments indicated that it has the properties expected of centre B. The signal shows changes on reduction of centre A (including a shift of the g = 2.065 peak),